Chromatography, in all its forms, is a technique used to separate a mixture of compounds or elements into its individual components. All chromatographic processes consist of two basic segments: a mobile phase and an immobile phase. The mobile phase moves through a capillary or tube, called a column, containing the immobile phase, and the sample to be separated into its basic components is injected into the mobile phase moving through the column. As the sample is swept forward by the mobile phase, the sample components are either adsorbed on the surface of the immobile phase (if the immobile phase is a solid) or dissolved in the immobile phase (if it is a liquid). As the mobile phase continues passing through the column, components of the sample are continuously desorbed back into that mobile phase. This adsorption-desorption process continues throughout the length of the column, each sample component in the mobile phase thus moving through the column at a different rate, depending primarily upon its attraction for the immobile phase. The components therefore separate as they pass through the column and emerge at the other end of the column at different times.
In liquid chromatography, the sample which is to be analyzed is a combination of liquid components which are carried through the column in the liquid phase by a mobile phase comprising a liquid carrier stream. The immobile phase in liquid chromatography generally comprises a solid in the form of uniform or other particles packed in the column.
In gas chromatography, the sample to be analyzed comprises volatile components which are carried through the column in a gaseous state by an inert mobile phase in this case, called the carrier gas. The immobile phase is either a solid in the form of uniform or other particles, or a thin film on either the particles and/or the column walls.
Therefore, under proper gas chromatographic conditions, various components of the gas sample are spacially separated by the above-described process of selective adsorption and desorption, so that the separated gas constituents issue from the end of the column in sequential order corresponding to their relative volatility, their molecular weight or some other property affecting the degree of adsorption on the immobile phase or packing material in the column. As these separated gases emerge from the column they are normally passed through a suitable detector element which measures a property of the gas indicative of the character and/or amount present.
The immobile phase or packing most commonly used in chromatographic columns includes diatomaceous earth, alumina, glass beads, fluoro carbons, silica gel, and the like. Conventionally, the packing in whatever form is chosen for a particular chromatographic column, is poured into the column in granular form and compacted therein by vibration, tamping or the like. To operate these gas chromatographic columns an inert gas such as helium, argon or nitrogen acts as the carrier gas for the sample, and flows continuously through the column. The use of such an inert mobile carrier gas insures that the carrier gas does not react with either the sample or the immobile phase. The samples are introduced into the carrier gas either as a liquid or a gas. Usually, liquid samples generally on the order of ten microliters or less, are injected rapidly into a chamber which is maintained at a temperature that insures quick and complete vaporization of the sample.
The efficiency of these columns determines the length of time it takes in order to perform a given analysis. That is, columns having high efficiencies can be of shorter length than columns having low efficiencies, and with the carrier gas flowing at the same rate the analysis can be performed much more readily. It is therefore desirable to improve the efficiency of a column for two principal reasons, i.e. that a column of high efficiency can perform an analysis much more rapidly, and that a column with high efficiency will have a greater ability to separate two sample components (or to resolve them) and hence the capability of analyzing products more precisely than will a column of standard efficiency having an equivalent length.
The efficiency of chromatographic columns is generally expressed in terms of theoretical plates, which is simply a number of theoretical plates per unit of length necessary to effect resolution. As a component in a sample is moved through the column by a carrier gas, the velocity at which the component is traveling, the dimensions of the column, the medium through which it travels and the packing density will have a direct influence on the column efficiency. As the packing density increases, however, there is a commensurate and undesirable pressure drop increase across the packed portion of the column.
Many techniques have heretofore been developed for packing chromatographic columns. Thus, for example, U.S. Pat. Nos. 2,845,136 and 3,164,980 each describe packing techniques wherein the packing material is poured into the chromatographic column and compacted therein by vibration, tamping or the like. Also, U.S. Pat. Nos. 3,248,856; 3,522,172; 3,692,669 and 3,796,657 disclose packing techniques wherein the packing medium is fluidized by various techniques and then allowed to settle to form a packed bed. In "GAS CHROMATOGRAPHY 1960" edited by R. P. W. Scott, published by Butterworths (London) at pages 240 et seq., the effect on packing structure of various modes of vibration, tapping, rotations or combinations thereof is disclosed.
"A STUDY OF PACKED CAPILLARY COLUMNS," by V. G. Berezkin et al, Journal of Chromatography, 99 (1974) pgs. 111-122, discloses the packing of columns by simply tapping the packed column lightly by hand or by moving a mechanical vibrator along the column. The authors also disclose the filling and packing of such columns utilizing a combination of an inert gas flow and low-frequency (50-100 Hz) electromechanical vibrators located at several positions along the column.
In "PERMEABILITY AND PREPARATION OF MICRO-PACKED COLUMNS," by J. A. Rijks et al, Chromatographia, Volumn 8, No. 9 September 1975, pages 482 et seq., the authors disclose simultaneously filling and packing a micropacked column (i.e. having a particle/column diameter rate between 0.1 and 0.3) with the use of gradually increasing pressure and ultrasonic vibrations. The authors also disclose that a hand vibrator held on the surface of the container is employed to assist in providing a continuous stream of particles to the column.
Finally, In "POTENTIALITIES OF MICROPACKED COLUMNS SOME APPLICATIONS IN PETROLEUM CHEMISTRY," C. A. Cramers, J. Rijks and P. Bocek, J. Chromatography, 65 (1972) 2937, a continuation of ultrasonic vibration and pressure is employed to simultaneously fill and pack a chromatographic column. This article also notes that at the start of the procedure the pressure increase shall be very gradual, and continues that care should be taken to maintain a substantially constant pressure product across the packing to ensure a homogeneous packing density.
Despite the many techniques currently available, all of which employ various combinations of pressure and vibration, those skilled in the art still recognize the need for a packing technique which will maximize packing density while still imposing a relatively low pressure drop across the packed portion of the column.
Accordingly, it is an object of the present invention to provide a packing technique which enables high separation efficiencies to be obtained.
It is another object of the present invention to provide a packing method which enables maximum packing densities to be obtained while still maintaining a relatively low pressure drop across the column.
It is a still further object of the present invention to provide a packing method which is capable of increasing separation efficiency of chromatographic columns.